Consumers use paper wiping products, such as facial tissues, paper towels, and bath tissues, for a wide variety of applications. Facial tissues are not only used for nose care but, in addition to other uses, can also be used as a general wiping product. Consequently, there are many different types of tissue products currently commercially available.
In some applications, tissue products are treated with polysiloxane compositions in order to increase the softness of the product. Adding silicone compositions to a tissue can impart improved softness to the tissue while maintaining the tissue's strength.
In the papermaking industry, various manufacturing techniques have been specifically designed to produce paper products which consumers find appealing. Manufacturers have employed various methods to apply chemical additives, such as silicone compositions, to the surface of a tissue web. Currently, one method of applying chemicals to the surface of a tissue web is the Rotogravure printing process. A Rotogravure printing process utilizes printing rollers to transfer chemicals onto a transfer roll and then onto a substrate. Chemical emulsions that are applied to webs using the Rotogravure printing process typically require the addition of water, surfactants, and/or solvents in order for the emulsions to be printed onto the substrate. Such additions are not only costly but also increase drying time and add process complexity.
Another method of applying chemical additives to the surface of a tissue web is spray atomization. Spray atomization is the process of combining a chemical with a pressurized gas to form small droplets that are directed onto a substrate, such as paper. One problem posed with atomization processes is that manufacturers often find it difficult to control the amount of chemical and the placement or pattern of the chemical that is applied to a tissue ply. Thus, a frequent problem with spray atomization techniques is that a large amount of over-spray is generated, which undesirably builds upon machinery as well as the surfaces of equipment and products in the vicinity of the spray atomizer. Furthermore, over-spray wastes the chemical being applied, and comprises a generally inefficient method of applying additives to a tissue web. Additionally, lack of control over the spray atomization technique also affects the uniformity of application to the tissue web.
In view of the above, a need exists in the industry for improving the method for application of chemical additives to the surface of a paper web. While softening agents are exceptionally good at improving softness there are drawbacks to their use. Polysiloxanes are generally hydrophobic, that is, they tend to repel water. Tissue products treated with polysiloxane tend to be less absorbent than tissue products not containing polysiloxane. Hydrophilic polysiloxanes are known in the art, however, such hydrophilic polysiloxanes are more water soluble and hence when applied to a tissue sheet will tend to migrate more in the z-direction of the sheet than the hydrophobic polysiloxanes. This means that less polysiloxane is available on the surface of the tissue product at a given addition level. Hence, higher levels of hydrophilic polysiloxanes are required to achieve the same level of softness as hydrophobic polysiloxanes. Hydrophilic polysiloxanes are also usually sold at a cost premium to the hydrophobic polysiloxanes. Therefore, hydrophilic polysiloxanes tend to be less effective at softening and more costly to use than hydrophobic polysiloxanes.
Increased hydrophobicity in a paper product, such as a tissue, can adversely impact upon the ability of the wiping product to absorb liquids. Hydrophobic agents can also prevent bath tissue from being wetted in a sufficient amount of time and prevent disintegration and dispersing when disposed in a commode or toilet.
On the other hand, increasing the hydrophobicity of a paper web does provide various advantages. For example, by making the web hydrophobic, the fluid strike-through properties of the tissue product are improved. In other words, fluids absorbed by the web remain on the interior of the web and thus do not transfer to the hands of a user. Hydrophobic tissue products prepared using standard cellulose sizing agents are described in U.S. Pat. No. 6,027,611 issued to McFarland, et.al., and incorporated by reference herein. However, those skilled in the art will recognize the difficulties associated with using sizing agents to control hydrophobicity to a level acceptable for tissue products, the addition often resulting in products having unacceptably high levels of hydrophobicity. Furthermore, addition of sizing agents as described by McFarland, et.al., does not allow for regions of high and low hydrophobicity in the sheet but rather creates a uniformly hydrophobic sheet.
Hence, additives that are hydrophobic in nature can make it difficult to find a proper balance between improving the properties of a web through the use of the additive and yet maintaining acceptable absorbency and wetability characteristics.
It is known to add a wetting agent directly to a polysiloxane emulsion then topically apply the polysiloxane, wetting agent composition to the tissue sheet to mitigate the hydrophobicity caused by addition of the polysiloxane. While this perhaps reduces the overall hydrophobicity of the sheet it does not allow for making tissues having uniform polysiloxane coverage with alternating hydrophobic and hydrophilic regions.
It is also known to topically apply hydrophobic additives in discrete locations on a tissue sheet in conjunction with relatively large untreated areas of the sheet such that less than 50% of the surface of the sheet is covered with the additive. Such discrete placement of the additive on the tissue sheet is expected to provide regions of hydrophobicity and hydrophilicity. However, such discrete placement requires a majority of the tissue surface to not contain the additive. As a result, reduced product benefits, such as softness, are realized relative to a sheet having a high level of surface coverage. Furthermore, this process precludes use of hydrophobic additives prior to the tissue drying step.
U.S. Pat. Nos. 6,238,519 and 6,458,243 issued to Jones, et. al, describe the use of deactivated ketene dimer agents to reduce the hydrophobicity of sheets relative to those made with standard alkyl ketene dimers. While lower hydrophobicity is noted, the application precludes formation of specific regions of hydrophobicity and hydrophilicity, hence, the application of deactivated ketene dimers does not allow for fine tuning control of hydrophobic and hydrophilic properties.
Thus, a need also currently exists for tissue products and methods to prepare tissue products containing hydrophobic additives wherein the hydrophobic additive is present across a majority of the sheet surface, yet the benefits to the product are provided without increasing the hydrophobicity of the product beyond desirable limits.